With recent developments of communications networks, the frequency bands applied are now extended even to the microwave band. In this connection, dielectric ceramics are now applied to dielectric oscillators, microwave integrated circuit boards or impedance matching of various microwave circuits, etc.
Such dielectric ceramic articles are required to have a dielectric constant (hereinafter designated as E) of 10 or thereabout, and to be low in both the absolute values of a dielectric loss tangent (hereinafter designated as tan .delta.) and a temperature coefficient of resonance frequency (hereinafter designated as Tf). On the other hand, alumina ceramics are expected to become such dielectric ceramics as mentioned above, since it has an E of about 10 and a low tan .delta., and its raw material occurs abundantly. However, it has not yet been put to any practical use due to the fact that the absolute value of Tf is high, as expressed in absolute terms of about--60 ppm/.degree. C. To solve this, it has been proposed to add MgO, TiO.sub.2 and CaO to alumina in a certain porportion (Japanese Patent Kokai-Publication No. 53-149696). In Japanese Patent Application No. 59-32113 specification (now JP Patent Kokai-Publication No. 60-176967), the present inventors have also proposed an alumina ceramic composition comprising Al.sub.2 O.sub.3, CaO and TiO.sub.2 for the same purpose.
The conventional compositions are unsuitable for the purpose of obtaining articles having a substantially constant tan .delta. but varied Tf values, since there is a large tan .delta. variation depending upon a very slight change in the amount of additives, even though an article having a desired Tf value is obtained.